A lens used for a pair of spectacles is generally supported by a ring-shaped front frame thereof (see FIG. 1). In case that a user wears or handles the spectacles, an external force is often exerted upon the frame or the lens. In that case, depending upon a magnitude and/or a direction of the external force thus exerted thereupon, the lens and/or the frame are/is deformed, so that the lens is often disengaged from the frame. In particular, because the spectacles used in recent years are the ones in which the lens is made of plastic and the frame is made of metal, the spectacles are liable to deformation by the external force. Therefore, the lens may be easily disengaged therefrom even by a small external force exerted thereupon. It is needless to say that such a disengagement of the lens therefrom is undesirable.
In case that the spectacles are used for protecting a user's eyeball in order to give a top priority to safety (for example, the spectacles which are used in a hazardous environment, such as a factory), a lens holding mechanism shown in FIG. 2 is adopted. With the lens holding mechanism, the lens is never disengaged from the front frame thereof, even when some external force is exerted upon the spectacles. In this mechanism, the front frame 2 has a sectional structure with a relatively deep U-shaped groove 3 which is defined by a bottom wall 4a, a first side wall (wall on a side of a face) 4b, and a second side wall 4c (wall on a side opposite the face). The lens 1 has a circumferential edge having a square-shaped cross section, and the circumferential edge is fitted deep inside the U-shape groove 3. Therefore, the circumferential edge of the lens 1 is surely held between the side walls 4b, 4c on both sides of the lens 1, and the lens is not easily disengaged from the frame even when some external force is applied thereto.
The lens holding mechanism employing the U-shaped groove type shown in FIG. 2 is superior in safety; on the other hand, the lens holding mechanism is inferior in fashionability. The reason is that it is necessary to secure a deep engagement depth or part, for the lens, in the frame, that it is necessary to provide a structure in which the lens is supported by the both side walls from both sides of the lens, and that the front frame, therefore, can not help but become large, or thick.
In contrast with the lens holding mechanism of FIG. 2, a most popular lens holding mechanism having a good fashionability, in other words, having a small line diameter, is shown in FIG. 3. In this mechanism, the front frame 2 has a sectional structure with a V-shaped groove 5 which is defined by a first inclined bottom surface (bottom surface on a side of a face) 5a, and a second inclined bottom surface (bottom surface on a side opposite the face) 5b. The lens 1 has a circumferential edge having an engagingly locking convex stripe (="Yagen", V-shaped ledge) 1c which is V-shaped in cross section and which corresponds to the sectional configuration of the frame. The engagingly locking convex stripe 1c is fitted into the V-shaped groove 5 with a slight play P with a state in which the engagingly locking convex stripe 1c roughly contacts the inclined bottom surface (refer to the circumferential edge of the lens shown in a solid line in the figure). In case that the thickness (="koba", edge) of the lens 1 is large (in case that the degree of the spectacles is high), the width of the V-shaped engagingly locking convex stripe 1c of the lens is smaller than the thickness of the lens, as shown in the figure.
According to the lens holding mechanism shown in FIG. 3, it has an advantage that the line diameter of the front frame can be reduced by reducing the thickness of the frame with respect to the thickness of the lens.
However, the lens holding mechanism of FIG. 3 has such a problem that the lens is apt to become easily disengaged from the front frame when an external force is exerted thereon. An imaginary line of FIG. 3 shows a situation in which the lens 1 is disengaged from the front frame due to an external force "F" which is exerted upon the lens 1. Namely, it is conceivable that the lens 1 and the frame 2 are deformed by the external force "F", that a first inclined bottom surface 1a of the lens 1 is easily allowed to slide over the first inclined bottom surface 5a defining the V-shaped groove of the front frame 2 as a result, and that the lens 1 is dropped out from the front frame 2. Where an importance is placed on the fashionability of the spectacles, an attempt may be made to reduce the thickness of the front frame along its length and may be made to reduce the depth of the V-shaped groove thereof. As a result, the tendency of the lens falling or dropping out therefrom, will increase. However, even if the front frame is of a type of the V-shaped groove, it is possible to prevent the lens from dropping out therefrom by making the V-shaped groove sufficiently deep. Actually, many varieties of such front frames are commercially available. But where the V-shaped groove is made deeper, the line diameter of the front frame becomes thicker; accordingly, it lacks fashionability.
From a global point of view, spectacles have a very long history. Strangely enough, however, any countermeasure to prevent the lens of spectacles from falling or dropping out therefrom, particularly to prevent the lens thereof from falling or dropping out therefrom in the lens holding mechanism which is of the type of the V-shaped groove shown in FIG. 3, has been scarcely taken.